How to find hidden drain

After giving my bike its weekly engine run i decided to clean it, so i took outside on the drive and went to look for cleaning rags etc, after searching around all i could find was a pink bed sheet i had been given ,so i spent some time ripping that to handy sizes.
Armed with cleaning kit i went to the bike to find her leaking petrol (i forgot to turn the fuel tap off) any way i changed the split pipe and got on with cleaning.
Wow she looks good, so start her up to get rid of any moisture, the first kick she spits back and catches fire ( some petrol must have leaked into the mag) a frantic few seconds smothering the flames with my polishing rag soon has the fire out, but the rag is smoldering and hot so i chuck it on the ground, not even thinking about the rag i turn my attension to my bike, any damage? well i am lucky every thing looks fine, i am just about to stand up when there is a BOOMFF noise behind me, i turns around and there just 2 yds away is a manhole
with smoke waffting out of it, it must have been hidden under the pebbles.

It was not my fault i possitioned my bike well away from (known) drains, heck i must have pipes each way and every way under my property, any way i did not get burned and my bike is fine so all in all a good day, apart from a little more co2.

Hey, if tribdog, Wolram, and I host a PF gathering, who wants to come? Should I start a poll?

If you host such a gathering, I will arrange for insurance via Lloyd's of London. Lloyd's "names" certainly will not have heard of you three, and will offer insurance cheap. Then, when the inevitable happens - I'll cash in.